1. Field of the Invention
The present invention relates to an apparatus of producing a silicon single crystal and a method for producing a silicon single crystal that can provide high-quality silicon single crystals having designated crystal properties based on critical control of surface position of silicon melt during pulling the silicon single crystals from the silicon melt by the Czochralski method.
Priority is claimed on Japanese Patent Application No. 2010-277212, filed Dec. 13, 2010, the content of which is incorporated herein by reference.
2. Description of Related Art
Conventionally, silicon single crystals have been produced through various methods. The Czochralski method (hereafter referred to as CZ method) is the most representative method for producing silicon single crystals. In the growth process of a silicon single crystal by the CZ method, a silicon melt is formed by melting polysilicon in a crucible. Then, a seed crystal is dipped in the silicon melt and is pulled up with a predetermined pulling speed while rotating the seed crystal with a predetermined rotation rate. As a result, a silicon single crystal of columnar shape is grown below the seed crystal.
In a silicon single crystal grown by the CZ method, species and distribution of the defects depend on the ratio of the pulling rate V of the silicon single crystal and thermal gradient G in the silicon single crystal along the growth direction. The ratio is hereafter referred to as V/G.
Under large V/G conditions, the silicon single crystal is in excess of vacancies, resulting in generation of small voids (defects generally called COPs) as agglomerates of the vacancies are formed in the crystal. On the other hand, under small V/G conditions, the silicon single crystal is in excess of interstitial silicon atoms, resulting in generation of dislocation clusters as agglomerates of the interstitial silicon atoms. Therefore, in order to grow a crystal that does not include COPs and dislocation clusters, it is necessary to control the radial distribution and axial distribution of V/G in an appropriate range.
With respect to the radial distribution, pulling rate V is constant in any positions along the radial direction in the single crystal. Therefore, it is necessary to design a structure of high temperature zone (hot zone) in the CZ furnace such that the thermal gradient G is within a predetermined range. With respect to axial distribution, G depends on the pull length of the single crystal. Therefore, it is necessary to change V along the lengthwise direction of the single crystal so as to control V/G in a predetermined range.
Currently, mass production of COP-free and dislocation clusters-free crystals is realized even in production of silicon single crystals of 300 mm in diameter based on the control of V/G.
However, even though a silicon single crystal that does not include COPs and dislocation clusters is grown by controlling V/G, a silicon wafer obtained from the crystal does not have a homogeneous property throughout the wafer plane, but includes a plurality of regions that exhibits different behaviors under a heat treatment. For example, when V/G is varied from COP generation conditions to dislocation cluster generation conditions, three regions consisting of so called OSF region, Pv region, and Pi region appear with decreasing V/G between the COP generation region and the dislocation cluster generation region.
The OSF region denotes a region which includes platy oxygen precipitates (OSF nuclei) under an as-grown state (a state at which the single crystal is not subjected to any heat treatment after the growth of the crystal) and generates OSFs (Oxidation Induced Stacking Faults) when the crystal is subjected to thermal oxidation. A Pv region denotes a region that includes oxygen precipitation nuclei under an as-grown state and easily generates oxygen precipitates when the crystal is subjected to two step heat treatment at low temperature and high temperature (for example, at 800° C. and 1000° C.). A Pi region denotes a region that is almost free of oxygen precipitation nuclei under an as-grown state and hardly generates oxygen precipitates even when the crystal is subjected to heat treatment.
There is a demand to provide high-quality silicon single crystal in which the above-described Pv region and the Pi region are formed as distinguishable regions (hereafter, such crystal is referred to as a PvPi crystal). It is revealed that precise control of V/G is required to grow a PvPi crystal. For example, it is necessary to control the fluctuation of V/G within a range of ±0.5% during the growth of a PvPi crystal.
In general, V/G is controlled by controlling the pulling rate V. With regard to control of V/G, it is known that thermal gradient G during pulling the silicon single crystal is largely affected by a distance (spacing) between the melt surface of the silicon melt and the heat-shield that is disposed to face the melt surface. In order to control the V/G to be in the growth conditions of a designated defect-free region, it is necessary that the distance between the melt surface and the heat-shield remain constant. On the other hand, it is necessary to lift up the crucible since the amount of the melt decreases in accordance with the progressive pulling of the silicon single crystal.
Conventionally, a volumetric loss of silicon melt as a result of pulling a silicon single crystal was calculated, and elevation of the crucible was calculated based on the volumetric loss of the silicon melt and the inner diameter of the crucible. However, it is difficult to calculate the loss of silicon melt precisely because of a change of dimension of the crucible due to deformation of the crucible at high temperature and because of error of measurement of the inner diameter of the crucible. Therefore, relative position of the melt surface and the heater is not stable. Therefore, in order to produce silicon single crystals having designated defect region by controlling V/G, it is necessary to measure a position of the surface of the silicon melt precisely during pulling of the silicon single crystal, and control the elevation of the crucible based on the measured value.
As an example of a method for precisely measuring the surface position (surface level) of the silicon melt, Japanese Examined Patent Application, Second Publication No. H3-31673 (Japanese Patent No. 1676655) describes the following method. A rod made of a refractory material such as quartz is attached to the heat-shield that covers the silicon melt such that the rod is attached to end portion facing the silicon melt. The position of the surface of the silicon melt (hereafter, referred to as melt-surface position) is determined by detecting the contact of the rod with the melt-surface.
As an alternative example, in the method described in Japanese Examined Patent Application, Second Publication No. H5-59876 (Japanese Patent No. 2132013), contact of the seed crystal and the melt surface is detected, and the surface level of the melt is determined relative to the contact position.
However, in the above-explained conventional methods, it has been difficult to detect the precise position of the melt surface at the time of starting the crystal pulling because of irregular lengths of seed crystals, fluctuation of melt-surface due to rotation of the crucible or the like. In addition, it is impossible to detect the position of the melt surface in real time during the pulling process.
In order to solve the above-described problems, an object of the present invention is to provide a method for producing a silicon single crystal and an apparatus of producing a silicon single crystal that enable production of silicon single crystals of high quality while detecting precise surface position of the silicon melt throughout the pulling process of the silicon single crystal including starting of the pulling and intermediate stage during continuing the crystal pulling.